Electroacoustic device



Dec. 15, 1936. H. F. OLSON ET AL I ELECTROACOUSTIC DEVICE Filed Dec. 14,1952 2 Sheets-Sheet 1 INVENTORS #4511) 0150 BY FfiA/YK MASS/1 ATTORNEYH. F. OLSON ET AL ELECTROACOUSTIC DEVICE Dec. 15, 1936.

14, 1952 2 Sheets-Sheet 2 JL Filed Dec.

. r arm 0 fl/ m INVENTORS HARRY F OLSU/V mA/v/r mas/4 A'TTCSRNEYPatented Dec. 15, 1936 UNITED STATES PATENT OFFICE ELEGTROACOUSTICDEVICE Application December 14, 1932, Serial No. 647,132

6 Claims.

This invention relates to electro-acoustic devices and has for itsprincipal object the provision of an improvedelectro-acousticdevicewhichhasa substantially constant response over acomparatively wide range of operating frequencies.

Another object of the invention is the provision of a telephone receiverwhich produces at the ear a substantially constant response.

A further object of the invention is the provision of an improvedelectro-acoustic device of the type wherein a single element functionsboth to carry the electrical current which actuates the device as wellas generating the acoustic pressure.

A further object is the provision of an improved telephone receiver ofthe ribbon type.

While the improved device herein described possesses particular utilityin connection with testing high fidelity transmission systems andmonitoring in broadcasting stations, sound picture and phonographrecording and the like, it will be apparent that it has importantadvantages in other cases where extreme accuracy of response isdesirable. As utilized in practice, its maximum variations in responsebetween 30 to 6,000 cycles is about 2 /2 db. When utilized as areceiver, its response above 6,000 cycles is purposely made to drop 7db. from the maximum at 8,000 cycles in order to compensate for anincrease in the ear sensitivity resulting from standing waves being setup in the ear cavity. From 8,000 to 10,000 cycles, its response issubstantially constant.

The invention will be better understood from the following descriptionwhen considered in connection with the accompanying drawings, and itsscope will be pointed out in the appended claims.

Referring to the drawings:

.Fig. 1 is an exploded view of a device constructed in accordancce withthe invention.

Fig. 2 is a sectionalized perspective view of the mechanism of thedevice,

7 .Fig. 3 illustrates the device as it appears with the front of thecasing removed, Fig, 4 is a perspective view of the device,

Figs. 5 to Tare explanatory diagrams for illustrating the effect ofvarious constructional features on the response of the device atdifferent frequencies, and

Fig. 8 illustrates a modified embodiment wherein means are provided forvarying the size of-the cavity at the rear of the diaphragm orvibratory. member.

The device includes a magnetic yoke provided with, pole shoes [I and I2and arranged to receive between its polar extremities an insulationmember I 3 which forms a support for the operating mechanism and alsoprovides a cavity It at the rear of the vibratory diaphragm I5.Interposed between the diaphragm I5 and the cavity or chamber M is amember It. This member serves to partition off the enclosure behind thevibrating diaphragm into two cavities. Several small holes are placedinto the member I6 of such dimensions that their inertance is such thatresonance is obtained with the stiffness of the air in the cavity l t atthe same frequency that the vibrating diaphragm and its air chamberresonate, and acts to smooth out the response curve as shown in Fig. 6and to be described later. Several small holes are placed into themember It instead of one larger hole in order to increase the resistivecomponent and thereby obtain a broader resonance.

The diaphragm I 5 is mounted on a strip II preferably made of empiresilk about .002 inch thick and cut away at its center in order todecrease the efiective mass of the vibrating system. The diaphragm I5 iselectrically insulated from the pole shoes II and I2 and is clamped atone of its ends between members I8 and I9 and 3.1; the other of its endsbetween members 20 and Electrical leads 22 and 23 connected to theopposite ends of the ribbon I5 either through the clamping members I8and 20 or in other suitable manner are provided for supplying operatingcurrent to the device.

In the assembly of the apparatus, the magnetic structure In is moldedinto a bakelite shell 24 which serves to form this structure into arigid unit and to provide a suitable exterior surface for the main bodyof the device. It will be apparent that the pole shoes I I and I2 areclamped to the insulation member I3 by means of screws 25 and that thenon-magnetic bridging members I9 and M are insulated from the pole shoesII and I2 by the insulation washers 26 and 21 and by the insulationcollars and Washers interposed between the bridging members I92I and thescrews 28 by which these members are clamped to the pole shoes. Aspreviously indicated, the ends of the ribbon I5 are clamped to theterminal blocks is and El by means of members I8 and 20, suitable meanssuch as members 29 and 30 being provided for clamping the edges of theempire silk support I! to the pole shoes. As indicated by Fig. 4, an earcap 3| is provided at the front of the casing in which the working partsare mounted and a suitable support 32 is provided. The

' the left.

ear cap has been made of molded rubber having an undercut projecting lipwhich easily conforms to the contour of the ear when placed against it,thus serving to prevent leakage of air between the vibrating diaphragmand the ear.

From what has been said, it will be apparent that the device consistsfundamentally of a magnetizing unit, a diaphragm mounting and an earcap. It is similar in some respects to the highquality microphone of theribbon type but differs therefrom in other respects.

In the case of the microphone, the ratio of the velocity of theconductor to the pressure in the sound wave must be independent offrequency to obtain a system in which the sensitivity is independent offrequency. The mechanical system in the microphone is mass controlled,that is, the mechanical impedance is proportional to the frequency. Tomaintain constant velocity of the ribbon, the acoustic system isdesigned so that force available for actuating the ribbon isproportional to the frequency over the working range of the microphone.In the case of the receiver, we must supply constant sound pressure tothe ear cavity for constant voltage applied to the receiver. If weassumethat the ear cavity presents a constant acoustic capacitance to thereceiver, the ratio of amplitude of the ribbon to the applied voltageshould be independent of the frequency. It is well known that theimpedance of the ear cavity presented to the receiver is not a purecapacitance, particularly at the higher frequencies, due to standingwave systems between the receiver and portion of the ear cavity, andalso due to absorption which results in a resistive component. However,these factors Vary from person to person. For this reason it seems thata receiver in which the ratio of amplitude to the applied voltage isindependent of frequency meets the actual conditions about as accuratelyas any other characteristic.

In order that the pressure in the ear cavity shall be independent of thefrequency for constant voltage applied to the receiver, the ratio of theamplitude of the ribbon diaphragm to the applied voltage must beindependent of the frequency. This is equivalent to stating that thesystem must be stiffness controlled. There are a multitude of ways inwhich this may be accomplished. The discussion which follows takes intoconsideration one system which will satisfy the requirements outlinedabove.

A system employing a ribbon diaphragm and and acoustic capacitance isshown in Figure 5 at The equivalent circuit of this system is shown inFigure 5 at the right. The velocity of the ribbon is given byexpression.

where F: force due to the current,

m=mass of the ribbon,

r= mechanical resistance,

V C 2 s2 p: density of air A: area of the ribbon, p: velocity of soundV=volume of the cavity, to: 211') f: frequency The amplitude in terms ofthe velocity X= The amplitude response of the system is shown at thecenter of Figure 5; It will be seen that the excessive response at theresonance frequency is objectionable. To maintain constant amplitude inthis system, either the resonant frequency must be located outside theoperating range, or a large amount of resistance must be used in thecircuit and the resonant frequency placed near the upper limit of theoperating range. The primary disadvantage of either of these systemslies in their relative insensitiveness.

In general in any system designed to yield uniform response efiicientlyover a large frequency band, it is necessary to resort to more than oneresonant circuit. A system which accomplishes this result is shown inFigure 6. It is this type of system which has been adopted for theribbon receiver.

In this system, the velocity of the ribbon is given by Y F: force duetocurrent,

m=mass of the ribbon,

r1=damping resistance of the ribbon, C1=compliance of first cavity,

C2: compliance of second cavity,

r2: resistance of the aperture, m2: equivalent mass of the aperture to:21rf frequency The amplitude in terms of the velocity is We may adjustthe constants of the system so. that the amplitude per unit force willbe practically independent of the frequency. The amplitude for a certainset of constants is shown at the center of Figure 6.

It was found very important to prevent all leakage of air from the frontto the back of the ribbon in order to preserve the low-frequencysensitivity of the receiver. An aluminum ribbon .00025" thick and 13/64wide cemented along its edges to a mounting of empire silk .002" thickcomprises the vibrating system. A window is cut out in the silk behindthe ribbon in order to decrease the effective mass of the diaphragm.

In order that the entire surface of the ribbon will vibrate in phase, itis necessary to corrugate it. Satisfactory corrugations are formed bypressing the mounted ribbon on a group of parallel wires .014" diameter,spaced 35 to the inch; the axis of the ribbon being kept parallel to thewires.

When the ribbon is clamped in its mounting it is installed withoutstretching. The controlling stiffness of the system is obtained by theenclosed volume of air behind the ribbon. With a fiat piece placedacross the bottom face of the pole pieces, an enclosure of 0.42 cc.remains behind the ribbon.

This stiffness resonates with the ribbon at about 8000 cycles. In orderto eliminate the excess response at resonance and increase thesensitivity, a second air chamber having a volume of .38 cc. is coupledto the first chamber thru 5 holes .012" diameter, drilled thru a brassplate 1/64" thick.

The frequency-response characteristic of the receiver is shown in Figure7.

The necessity for preventing air leakage from the front to the back ofthe ribbon has already been mentioned. It is equally important thatthere is no air leakage from the ribbon to the atmosphere when it isplaced against the ear. In order to have a good seal between the ear capand the ear, the cap was made of rubber molded into the shape shown. Inassembling the ear cap to the receiver, a layer of sealing compoundcomposed of wax and oil was smeared over the pole pieces beforefastening the cap. This compound effectively prevents leakage betweenthe ear cap and pole shoes.

The two ribbons in the receivers making up the head set are connected inseries and a transformer is provided for stepping up the impedance to2000 ohms. The resistance of both ribbons in series is .065 ohms;therefore, it is necessary to use fairly heavy leads from the receiversto the transformer in order to prevent loss of power in the cable. Fourfeet of parallel, finely-stranded cable equivalent to #10 B. & S. gaugecomprises the phone. cord to which the transformer is attached. Asatisfactory transformer has been made having 10 turns of #16 enamelledcopper for the primary and 1600 turns of #36 enamelled copper for thesecondary (the primary is wound between two halves of the secondary inorder to reduce the secondary capacity). The core consists of a stack ofsmall nicoloi laminations having an outside dimension 1 x 1 The absolutesensitivity of each receiver at 1000 cycles has been measured as 1.3bars into a 4 cc. cavity per milliwatt electrical input.

As will be readily understood, the advantages of the invention are notlimited to the specific structure disclosed but may be realized withdifferent modifications of this structure. Figure 8, for example,illustrates a modification wherein the size of the cavity at the rear ofthe diaphragm I5 may be adjusted by means of a rotatable control member33. By controlling the size of the cavity behind the diaphragm, theresonant frequency of the system may be adjusted to any value within thelimits of motion of the member 33. This construction provides a receiverhaving a very high output at any single frequency within the range ofthe unit. Other modifications of the invention will be apparent in viewof the foregoing description.

We claim:

1. An electro-acoustie device including means for producing a magneticfield, an insulating support impervious to air mounted in said field andprovided with a central aperture, a stiffness controlled diaphragmarranged on said support to cover said aperture, and electrical leadsconnected to said diaphragm.

2. An electro-acoustic device including means for producing a magneticfield, means arranged to form a chamber at one side of said field, aflexible insulating strip mounted in said field and provided with anaperture, and a stiffness controlled diaphragm coordinated with saidstrip to form a seal over the opening of said chamber.

3. An electro-acoustio' device including means for producing a magneticfield, an empire silk strip mounted in said field and provided with anaperture, a diaphragm arranged to cover said aperture and provided withcorrugations extending transversely of said field, and means supportingrigidly the diaphragm at the ends of the corrugations.

4. An electro-acoustic device including means for producing a magneticfield, longitudinally corrugated aluminum diaphragm having a thicknessof substantially .00025 inch mounted in said field, means forming anacoustic chamber having an opening at one side of said diaphragm,

and a flexible insulating support combined with said diaphragm to form aseal to said opening whereby the operation of said device is stiffnesscontrolled.

5. An electro-acoustic device including means for producing a magneticfield, a ribbon diaphragm corrugated transversely of and mounted by itsends in said field, an air chamber mounted on one side of said ribbondiaphragm, a second similar chamber separated from the first by apartition, and holes in said partition.

6. An electro-acoustic device including means for producing a magneticfield, an empire silk strip mounted in said field and provided with anaperture, a diaphragm arranged to cover said aperture and provided withcorrugations ex.- tending transversely of said field, means supportingthe diaphragm at the ends of its corrugations and means arranged to forma chamber at one side of said diaphragm and said chamber divided intotwo parts by a partition provided with fine holes.

HARRY F. OLSON. FRANK MASSA.

